diff --git a/frontend/jest.config.js b/frontend/jest.config.js
new file mode 100644
index 0000000..86f88fb
--- /dev/null
+++ b/frontend/jest.config.js
@@ -0,0 +1,11 @@
+const { createDefaultPreset } = require("ts-jest");
+
+const tsJestTransformCfg = createDefaultPreset().transform;
+
+/** @type {import("jest").Config} **/
+module.exports = {
+  testEnvironment: "node",
+  transform: {
+    ...tsJestTransformCfg,
+  },
+};
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diff --git a/frontend/lib/api.test.ts b/frontend/lib/api.test.ts
new file mode 100644
index 0000000..5c81b06
--- /dev/null
+++ b/frontend/lib/api.test.ts
@@ -0,0 +1,181 @@
+import {
+  createSession,
+  uploadNotes,
+  startSession,
+  sendChunk,
+  stopSession,
+  getSession,
+  downloadTranscript,
+} from "./api";
+
+jest.mock("./supabase", () => ({
+  supabase: {
+    auth: {
+      getSession: jest.fn(),
+    },
+  },
+}));
+
+import { supabase } from "./supabase";
+
+const mockedGetSession = supabase.auth.getSession as jest.Mock;
+
+describe("api.ts", () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+    global.fetch = jest.fn();
+    mockedGetSession.mockResolvedValue({
+      data: {
+        session: {
+          access_token: "fake-token",
+        },
+      },
+    });
+  });
+
+  test("createSession sends POST with mode baseline", async () => {
+    (global.fetch as jest.Mock).mockResolvedValue({
+      json: async () => ({ id: "123", mode: "baseline" }),
+    });
+
+    await createSession("baseline");
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      "http://localhost:8000/api/sessions",
+      expect.objectContaining({
+        method: "POST",
+        headers: expect.objectContaining({
+          "Content-Type": "application/json",
+          Authorization: "Bearer fake-token",
+        }),
+        body: JSON.stringify({ mode: "baseline" }),
+      })
+    );
+  });
+
+  test("startSession sends POST to correct endpoint", async () => {
+    (global.fetch as jest.Mock).mockResolvedValue({
+      json: async () => ({ ok: true }),
+    });
+
+    await startSession("session-1");
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      "http://localhost:8000/api/sessions/session-1/start",
+      expect.objectContaining({
+        method: "POST",
+      })
+    );
+  });
+
+  test("sendChunk sends correct transformed payload", async () => {
+    (global.fetch as jest.Mock).mockResolvedValue({
+      ok: true,
+      json: async () => ({ partial_text: "hello world" }),
+    });
+
+    const result = await sendChunk("session-1", {
+      chunkIndex: 3,
+      audioB64: "abc123",
+      mime: "audio/webm",
+    });
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      "http://localhost:8000/api/sessions/session-1/chunks",
+      expect.objectContaining({
+        method: "POST",
+        body: JSON.stringify({
+          chunk_index: 3,
+          audio_b64: "abc123",
+          mime: "audio/webm",
+        }),
+      })
+    );
+
+    expect(result).toEqual({ partial_text: "hello world" });
+  });
+
+  test("sendChunk throws backend detail on failure", async () => {
+    (global.fetch as jest.Mock).mockResolvedValue({
+      ok: false,
+      status: 400,
+      json: async () => ({ detail: "Bad chunk" }),
+    });
+
+    await expect(
+      sendChunk("session-1", {
+        chunkIndex: 1,
+        audioB64: "bad",
+        mime: "audio/webm",
+      })
+    ).rejects.toThrow("Bad chunk");
+  });
+
+  test("getSession sends GET request", async () => {
+    (global.fetch as jest.Mock).mockResolvedValue({
+      json: async () => ({ id: "session-1" }),
+    });
+
+    await getSession("session-1");
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      "http://localhost:8000/api/sessions/session-1",
+      expect.objectContaining({
+        headers: expect.objectContaining({
+          Authorization: "Bearer fake-token",
+        }),
+      })
+    );
+  });
+
+  test("downloadTranscript sends GET request", async () => {
+    (global.fetch as jest.Mock).mockResolvedValue({
+      json: async () => ({ transcript: "done" }),
+    });
+
+    await downloadTranscript("session-1");
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      "http://localhost:8000/api/sessions/session-1/download",
+      expect.any(Object)
+    );
+  });
+
+  test("uploadNotes uses FormData and auth header", async () => {
+    (global.fetch as jest.Mock).mockResolvedValue({
+      json: async () => ({ ok: true }),
+    });
+
+    const file = new File(["pdf content"], "notes.pdf", {
+      type: "application/pdf",
+    });
+
+    await uploadNotes("session-1", file);
+
+    expect(global.fetch).toHaveBeenCalledWith(
+      "http://localhost:8000/api/sessions/session-1/notes",
+      expect.objectContaining({
+        method: "POST",
+        headers: {
+          Authorization: "Bearer fake-token",
+        },
+        body: expect.any(FormData),
+      })
+    );
+  });
+
+  test("omits Authorization header when logged out", async () => {
+    mockedGetSession.mockResolvedValue({
+      data: { session: null },
+    });
+
+    (global.fetch as jest.Mock).mockResolvedValue({
+      json: async () => ({ id: "123" }),
+    });
+
+    await createSession("context");
+
+    const [, options] = (global.fetch as jest.Mock).mock.calls[0];
+    expect(options.headers.Authorization).toBeUndefined();
+  });
+});
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